Retardation layer having a dispersion adapted to the active liquid-crystalline cell

ABSTRACT

The invention is in the field of retardation layers comprising high-molecular weight liquid-crystalline material for liquid-crystalline displays. The invention is directed to a retardation layer for a liquid-crystalline display comprising high-molecular weight liquid-crystalline material, wherein the dispersion has been adapted to that of the active liquid-crystalline cell by varying the mesogenic groups of the high-molecular weight liquid-crystalline material, so that the difference in dispersion between the active cell and the retardation layer in the wavelength area of 400-800 nm is not more than 0.1.

The invention is in the field of retardation layers comprisinghigh-molecular weight liquid-crystalline material. Such retardationlayers are used in displays. FIG. 1 is a schematic depiction of theconstruction of a display.

FIG. 1 shows a cross-section of a display comprising an active twistedlayer (4), the active cell, which can be switched on and off by means oftransparent electrodes (6), and a retardation layer (3), with substrates(2) disposed on either side of the two layers (3) and (4). At the outersides of the two outermost substrates there are polarisers (1), andunderneath the polariser situated under the active twisted layer ispositioned a mirror (5).

DESCRIPTION OF THE RELATED ART

In practice, the mirror can be omitted in some displays. The inventionis directed in particular to the retardation layer (3) of a display.Retardation layers serve to compensate for the undesirable birefringenceeffect that occurs in the active cell in the display. For theretardation layer use may be made of a twisted nematic layer composed oflow-molecular weight liquid-crystalline material such as is describedin, e.g., Kirk Othmer's Encyclopedia of Technology, 3rd ed. (New York:Wiley & Sons) Vol. 7, p. 728. Although low-molecular weightliquid-crystalline material gives good compensation when used, it isattended with the drawback of being low-viscous. For that reason, thelow-molecular weight material is sealed between inflexible substrates bymeans of spacers in order to attain a twisted, form-retaining structure.In other words, a closed, rigid cell has to be made.

Alternatively, use may be made of birefringent films, e.g., a film ofdrawn polymer such as a birefringent polycarbonate film. Such abirefringent polycarbonate film is described in Jap. J. Appl. Physics,Vol. 30, No. 4 (April 1991), 682-686. By using birefringentpolycarbonate films a liquid-crystalline display of reduced thicknessand weight may be obtained. However, said birefringent polycarbonatefilms fail to provide optimum contrast.

The reason for this poor contrast is as follows:

As stated above, retardation layers serve to compensate for theundesirable birefringence effect that occurs in the active cell of adisplay. This birefringence effect depends on the retardation value, theangle of twist, and the direction of twist of the layer ofliquid-crystalline molecules in the active cell of the display. Theretardation of a birefringent layer is defined as the product of thebirefringence value (Δn) and the layer thickness. At a given wavelength,the birefringence effect of the active cell of the display can becompletely compensated for by using a retardation layer that has equalretardation, and an equal as well as an opposite angle of rotationcompared with the active cell. For full compensation these conditionsshould apply for the entire visible part of the wavelength spectrum.This requirement can only be realised if the dependence of thebirefringence on the wavelength, also known as the dispersion, of thematerial of the retardation layer is equal to that of the LC materialused in the active cell of the display. This is not the case forbirefringent polycarbonate films. The dispersion of birefringentpolycarbonate films is lower than liquid crystalline active cells whichare commercially used. Therefore, their retardation can only be set (bysetting the layer thickness) to match the retardation of the active cellat 550 nm. As a consequence, over the rest of the visible wavelengtharea the retardation fails to match that of the active cell of thedisplay, especially in the wavelength area o f 400-550 nm the dispersionappears to be too low. This results in a less than optimal contrast.

In DE 39 25 382 A1 it is acknowledged that the optical properties of thecompensating film (i.e., the retardation layer) should have a wavelengthdependency which is substantially identical to that of theliquid-crystalline layer used for displaying information ( i.e. theactive liquid-crystalline cell). Further, DE 39 25 382 teaches that aretardation layer containing a liquid-crystalline polymer is moresuitable than a layer consisting of stretched polycarbonate when iscomes to the desired compensation.

However, DE 39 25 382 Al does not teach how the dispersion of theretardation layer can be matched very precisely with the dispersion of aspecific active liquid-crystalline cell.

DESCRIPTION OF THE INVENTION

In the present invention a retardation layer of high-molecular materialis provided which has a retardation virtually matching that of theactive cell over the whole visible wavelength area. Accordingly, theinvention is directed to a method for preparing a liquid-crystallinedisplay, which display comprises an active liquid-crystalline cell and aretardation layer containing a high-molecular weight liquid-crystallinematerial, wherein the dispersion of the retardation layer is adapted tothat of the active liquid-crystalline cell by varying the mesogenicgroups of the high-molecular weight liquid-crystalline material, so thatthe difference in dispersion between the active cell and the retardationlayer in the wavelength area of 400-800 nm is not more than 0.1,preferably not more than 0.03.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of the construction of a display.

FIG. 2 shows the dispersion (defined as the retardation at a certainwavelength divided by the retardation at 550 nm) for LC 3, LC 5, anactive cell used in the Sharp wordprocessor WD A 330 ("Sharp"), apolycarbonate film ("PC") and an active cell containing a commerciallyavailable liquid crystal mixture ZLI 4544 from Merck ("ZLI 4544").

FIG. 3 shows the dispersion for various LC materials according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

By high-molecular weight liquid-crystalline material are meant:relatively low-molecular weight liquid-crystalline polymers, oligomers,and liquid-crystalline glasses. The molecular weight forliquid-crystalline glasses and oligomers varies from 1000 to 4000, forliquid-crystalline polymers it varies 1000 to 20 000. High-molecularweight liquid-crystalline material has a higher mechanical strength thanlow-molecular weight liquid-crystalline material. Therefore, it is notnecessary to seal the liquid-crystalline material in a rigid cell.Because liquid-crystalline material is used, and the mesogenic groups ofliquid-crystalline material can easily be varied, it is possible toobtain a retardation layer which has approximately the same dispersionas that of the active cell.

The dispersion is defined here as the retardation (or the birefringence)at a certain wavelength divided by the retardation (or thebirefringence) at 550 nm.

It appears that the dispersion of a liquid-crystalline material can bevaried by the following measures:

By using mesogenic groups with large conjugated systems the dispersionof liquid-crystalline material is increased, whereas smaller conjugatedsystems lower the dispersity, especially in the wavelength area of400-550 nm. Usually, mesogenic groups have the following generalformula:

    -(CH.sub.2).sub.m --O--φ.sup.1 --(Q).sub.n --φ.sup.2 --R.sup.1

wherein:

m stands for an integer from 0-6,

Q stands for --C(O)--O--, --C═C--, --C═N--, --N═C--, --O--C(O)--,--C.tbd.C--or --N═N--,

R¹ stands for --O--R², --NO₂ --CN, --HC═C(CN)₂, --C(CN)═C(CN)₂ or --R²,

φ¹ stands for a substituted or unsubstituted cyclic, aromatic orheterocyclic compound having 4-10 carbon atoms,

φ² (stands for a cyclic, aromatic or heterocyclic compound having 4-10carbon atoms,

n stands for 0 or 1.

If for Q groups such as --C═C--, --C═N--, --N═C--or --C .tbd.C--areused, or if n is O, the mesogenic group has a large conjugated system.By using --C(O)--O--or --O--C(O)--the conjugation is decreased. Theconjugation can be further decreased by using --O--R² or R² for the R¹end group of the mesogenic group. If for φ¹ or φ² a non-aromatic cycliccompound is used, the dispersity will be lower than when aromaticcompounds are used.

By using mesogenic groups with polar moieties the dispersion of theliquid-crystalline material is increased. For instance, if mesogenicgroups according to formula 1 are used with --NO₂ as R¹ instead of O--R²or R², the dispersion is increased. Halogenation of the mesogenic groupalso gives an increase of dispersion.

When the dispersion of a commercially available active cell is known,the artisan can easily adjust the dispersion of the retardation layervia the measures described above. The birefringence at a certainwavelength can easily be measured with a refractometer, and frombirefringences at various wavelengths the dispersion can be calculated.The retardation of a commercially available cell can be measured withvarious optical techniques known to the artisan. From the retardation ata certain wavelength and the retardation at 550 nm the dispersion at acertain wavelength can be calculated.

For accurate matching of the dispersion of an active cell,liquid-crystalline material may be used wherein both mesogenic groupshaving a large conjugated system and mesogenic groups having a smallerconjugated system are present. By varying the ratio of the two kinds ofmesogenic groups the dispersion can be accurately matched with theactive cell.

Examples of the cyclic or aromatic compounds φ¹ and φ² include: ##STR1##wherein R³ stands for an alkyl group having 1-5 carbon atoms.

Examples of R² groups include:

--(CH₂)_(x) --O--C(O)--C(CH₃)═CH₂,

--(CH₂)_(x) --O--C(O)--CH═CH₂,

--(CH₂)_(x--CH) ₃,

--CH₂ --CH(CH₃)--(CH₂)_(x--CH) ₃,

--CH(CH₃)--(CH₂)_(x--CH) ₃, wherein x=1-14.

Some of these R² groups contain an asymmetrical carbon atom. The use ofchiral (exc usively laevorotatory or dextrorotatory) R² groups may beadvantageous in LCD retardation layers, as will be explained below.

It was found that the dispersion of high-molecular weightliquid-crystalline material is mainly dependent on the mesogenic group.A specific mesogenic group gives the virtually same dispersionirrespective of the liquid-crystalline polymer, oligomer or glass intowhich it is incorporated.

As mentioned above, high-molecular weight material has a highermechanical strength than low-molecular weight material. This makes itpossible to place the liquid-crystalline material between glasssubstrates having a thickness of 20-500 micrometers instead of thickglass substrates. The liquid-crystalline material may even be placedbetween or coated on flexible plastic substrates such as PET andpolycarbonate.

To obtain full compensation for the birefringence effect of the activecell, it is also necessary for the retardation layer to have an equal aswell as an opposite angle of rotation compared with the active cell. Atwisted structure is obtained by placing the liquid-crystalline materialbetween two orienting substrates, giving one of the substrates adifferent orientation direction from that of the other substrate.

Various techniques are known for making an orienting substrate. Forinstance, the substrate itself may be rubbed in a single direction. Thesubstrate in that case may be made of, e.g., polyimide, polyvinylalcohol, glass, etc. Alternatively, the substrate may be provided with athin orienting layer. This may be a thin polymer layer which can berubbed, e.g., polyimide, polyvinyl alcohol, etc. Alternatively, thisthin orienting layer may be a SiO_(x) layer evaporated at an angle ofless than 90°, usually of 60° or 86°. Generally, a substrate of poorflexibility, such as glass or quartz, is used for SiO_(x) evaporation.These orienting techniques are known to the skilled person and requireno further elucidation here. Of course, it is also possible to employother orienting techniques.

To control the direction of rotation of the director (to the left or tothe right) and/or to obtain an angle of rotation greater than 90°, theliquid-crystalline material is frequently mixed with a chiral material:the so-called chiral dopant. In principle, any optically active compoundmay be used to this end. As examples may be mentioned cholesterolderivatives and 4-(4-hexy.right brkt-top.oxy-benzoy.right brkt-top.oxy)benzene acid 2-octyl-ester. Ordinarily speaking, up to 5 wt. % of chiraldopant is employed in relation to the total amount of liquid-crystallinematerial. Alternatively, the liquid crystalline material itself may beprovided with chiral centres. Preferably, this is done by providing themesogenic group with a chiral chain (group R₂) or spacer, since in thisway the transition temperatures will hardly if at all be adverselyaffected. Examples of mesogenic groups with chiral chains have beendescribed above.

The angle of rotation of an STN display cell typically is 240° but maybe any other appropriate value. In the case of an angle of rotation of90° (or -90°), the film is generally called "twisted nematic." For aTFT-TN compensation layer an angle of rotation of 90° (or -90°) isrequired. If the angle of rotation is greater, the film is called"supertwisted nematic." In addition, this invention also concernsretardation layers with a smaller angle of rotation, from 0° (no twist)to 90° (or -90° ). For convenience these layers are also called "twistednematic" here. In the case of an angle of rotation of 0°, thearrangement of the liquid-crystalline layer will be uniform planar. Atangles of rotation exceeding 360° the structure goes through more thanone full rotation within a single layer. The length covered by thestructure in a full rotation is called the pitch. The invention is alsodirected to retardation layers having more than one pitch (even morethan 5 pitches).

The value of optical retardation (=Δn (birefringence) X d (thickness ofthe (S)TN layer) may be adjusted by choosing an appropriate value forthe thickness of the layer. This can be done by using spacers ofappropriate size. In general, glass spheres, polymer spheres or silicaspheres are used as spacers.

Alternatively, the high molecular-weight liquid-crystalline film can beplaced between the substrate of the display cell and another substrate.In a further embodiment of the invention the LC polymer film is placedbetween the polariser and a substrate. In these embodiments of theinvention a second substrate is not necessary and the thickness andweight of the retardation layer are reduced further.

The invention is further directed to a liquid crystalline displayobtainable by the methods described hereinbefore. Further, the inventionis directed to said liquid-crystalline display wherein both mesogenicgroups having a large conjugated system and mesogenic groups having asmaller conjugated system are present.

The invention will be further illustrated with reference to thefollowing unlimitative Examples.

EXAMPLES Example 1

Liquid-crystalline glasses were prepared from mesogenic group-containingepoxides and diamines.

Synthesis of LC glasses (general method):

A mixture of 1 eq. of diamine and 4 eq. of epoxy was heated for 5 hoursunder a nitrogen atmosphere at a temperature of 130° C. The melt wascooled down and dissolved in THF, and the solution of approximately 20%(m/M) was precipitated in a 10-fold excess of ethanol . The yields werein the range of 75 to 90%.

epoxide of cyanobiphenyl

A mixture of 39.0 g (0.20 mole) of hydroxycyanobiphenyl, 100 ml (1.25moles) of epichliorohydrin, and 0.44 g (2.4 mmoles) of benzyl trimethylammonium chloride was heated to 70° C. Next, a solution of 17 g (0.42mole) of sodium hydroxide in 100 ml water was dispensed in 3 hours.Following this addition there was one extra hour of stirring at 70° C.The reaction mixture was cooled to 20° C., and 200 ml of dichloromethanewere added. The organic layer was separated from the aqueous one andwashed with, successively, NaCl solution (twice) and water (twice).After drying on magnesium sulphate and concentration by evaporation thecrude product was converted to the crystallised form from 450 ml ofmethanol. The yield was 38.30 g (76%).

The epoxide of cyanobiphenyl was used to prepare an LC glass (LC 1) bythe general method for the synthesis of LC glasses specified above,using m-xylylene diamine (m-XDA), ex "FLUKA®" (Fluka Chemie AG, St.Gallen, Switzerland. The molecualar weight was found to be 1140, Tg:64°/70° C., Tc: 127° C.

epoxide of methoxyphenyl benzoate

Preparation of 4-methoxyphenol-4'oxybenzoate

74.5 g (0.6 mole) of 4-methoxyphenol, 55.3 (0.40 mole) of hydroxybenzoicacid, and 1.24 g (20 mmoles) of boric acid were dissolved in 750 ml oftoluene. Next, 2.0 g (20.4 mmoles) of H₂ SO₄ were added dropwise, andthe mixture was refluxed the formed water being distilled offazeotropically. The toluene was evaporated, and the reaction product waswashed twice in 200 ml of diethyl ether/petroleum ether (1:1 (V:V)). Theproduct was twice converted to the crystallised form from 400 ml ofacetonitrile and then dried. The yield was 56.1 g (49%).

A mixture of 42.0 g (0.17 mole) of 4-methoxyphenol-4'oxybenzoate, 100 ml(1.25 moles) of epichlorohydrin, and 0.35 g of benzyl trimethyl ammoniumchloride was heated to 70° C. Next, a solution of 6.4 g (0.16 mole) ofsodium hydroxide in 32 ml of water was dispensed in 2 hours. Followingthis addition stirring continued for 2 more hours at 70° C. The reactionmixture was cooled to 20° C., and the organic layer was separated fromthe aqueous one and washed with 50 ml of water. The excessepichlorohydrin was removed by means of vacuum evaporation at atemperature below 50° C. The residue was dissolved in 250 ml ofbutanol/toluene (1:2 (V:V)) and stirred for 1 hour at 30° C. in thepresence of a 20%-solution of NaOH (1.49 g). The organic layer waswashed with water several times. After vacuum evaporation the crudeproduct was twice converted to the crystallised form from methanol. Theyield was 28.5 g (55%).

A liquid-crystalline glass (LC 2) was prepared by the general method forthe synthesis of LC glasses specified above using methylene diamine, ex"FLUKA®" (Fluka Chemie AG, St. Gallen, Switzerland). The molecularweight turned out to be 1398, Tg: 66°/72° C., Tc: 127° C.

Example 2

Liquid-crystalline polyethers were prepared from mesogenicgroupcontaining epoxides and mesogenic group-containing diols.

Synthesis of LC polyethers (general method):

To a mixture of OH-containing compound and 5% of BF₃ Et₂ O indichloromethane there was slowly added dropwise, at room temperature,epoxide dissolved in dichloromethane. In the case of acrylate alcoholsbeing used, a pinch of "IONOL®" (Shell Chemical Corp., New York, N.Y.)was added. The polymerisation mixture was stirred overnight and thenneutralised with solid CaO. After one hour the CaO was filtered off. Thepolyether was precipitated in ether, washed with ether, and dried undervacuum. The yield was 75-90%.

The epoxide of methoxyphenyl benzoate was used to prepare aliquid-crystalline polyether (LC 3) by the general method for thesynthesis of LC polyethers specified above, using methoxyphenyl-(2,3dihydroxypropyloxy)benzoate with an epoxy/OH ratio of 5:1. The diol wasprepared in the same manner as the hexyloxy analogon in EP-A2-0 550 105.The molecular weight turned out to be 2984, Tg: 46°/52° C., Tc: 146° C.

epoxide of nitrophenyl benzoate

Preparation of 4-nitrophenyl 4'oxybenzoyl epoxypropyl ether

To a solution of 56 g (1 mole) of potassium hydroxide in 225 ml of waterwere added 69 g (0.5 mole) of p-hydroxybenzoic acid. To this solutionwere slowly added dropwise, at room temperature, 42 g (0.55 mole) ofallyl chloride. Following the addition of the allyl chloride there wasrefluxing for a further 18 hours. After cooling the reaction mixtureseparated into two layers. A solution of 28 g (0.5 mole) of potassiumhydroxide in 240 ml of water was added, and the whole was heated until ahomogeneous reaction mixture had formed. After renewed cooling andacidification with concentrated hydrochloric acid 4(allyloxy)benzoicacid was precipitated. This product was recrystallised from 250 ml ofglacial acetic acid. 32 g (0.18 mole) of the dried 4(allyloxy)benzoicacid were dissolved in 150 ml of thionyl chloride, whereupon 2 drops ofdimethyl formamide were added and the whole was boiled with refluxing.Thionyl chloride was distilled off, and after being cooled the residuewas incorporated into 100 ml of dry dichloromethane. After filtrationthe dichloromethane solution was added, with vigorous stirring, over 1hour and at a temperature of 5°-10° C., to a solution of 23 g ofnitrophenol (0.166 mole) in a mixture of 135 ml of dichloromethane and34.2 ml of pyridine. There was 2 hours of afterstirring at roomtemperature. 250 ml of dichloromethane were added to the reactionmixture; the whole was washed twice with dilute hydrochloric acid andthen washed until neutral. After distilling off of the solvents theresidue was converted to the crystallised form from methanol. The yieldwas 37.6 g (70%).

10 g (33 mmoles) of 4-nitrophenyl 4'oxybenzoyl allyl ether weredissolved in 50 ml of dichloromethane, and 11.2 g (45.5 mmoles) ofm-chloroperbenzoic acid were added under nitrogen. After 24hours'stirring at room temperature 250 ml of dichloromethane were added,and the solution was washed with sodium carbonate solution and then withwater until neutral. After drying and distilling off of the solvent theresidue was converted to the crystallised form from 250 ml of ethanol.The yield was 8.1 g (77%).

The epoxide of nitrophenyl benzoate was used to prepare aliquid-crystalline polyether (LC 4) by using the general method for thesynthesis of LC polyethers specified above, using nitrophenyl-(2,3dihydroxypropyloxy)benzoate with an epoxy/OH ratio of 5:1. The diol wasprepared in the same manner as the hexyloxy analogon in EP-A2-0 550 105.The molecular weight turned out to be 3173, Tg: 58°/63° C., Tc: 130° C.

Epoxide of methoxycyclohexyl benzoate

4(2,3 epoxypropyl oxy)phenyl 4'methoxycyclohexyl carboxylate

76 g (480 mmoles) of 4 methoxycyclohexane carboxylic acid (cis/transmixture) were boiled for 7 hours with refluxing in 350 ml of thionylchloride to which several drops of dimethyl formamide had been added.The obtained 4 methoxycyclohexane carboxylic acid chloride was composedalmost completely of the trans compound. After distilling off of thethionyl chloride the residue was incorporated into 75 ml of drytetrahydrofuran. At a temperature of from 0° to 5° C. this solution wasslowly added dropwise to a solution of 158.4 g (1440 mmoles) ofhydroquinone in 650 ml of tetrahydrofuran and 375 ml of pyridine. When,after this addition, the mixture had attained room temperature, it waspoured onto ice and concentrated sulphuric acid. Extraction withdichloromethane, evaporation of the dichloromethane, and, in succession,conversion of the evaporation residue to the crystallised form from anethanol-water mixture and from toluene gave a yield of 24.45 g (20%) ofpure trans 4 hydroxyphenyl 4'methoxycyclohexyl carboxylate. 24.3 g (97mmoles) of the above compound were boiled, with refluxing, for 24 hourswith 17.6 g of allyl bromide (145 mmoles) and 13.4 g (97 mmoles) ofpotassium carbonate in 350 ml of methylethyl ketone. After cooling thereaction mixture was poured into 1 .right brkt-top.of ice water, whichwas extracted with the aid of diethyl ether. After drying andevaporation of the diethyl ether 28.9 g (97%) of 4 allyloxyphenyl4'methoxycyclohexyl carboxylate were obtained. To 28.7 g (99 mmoles) ofsaid compound in 250 ml of dichloromethane there were added 32.9 g ofchloroperbenzoic acid, and the mixture was stirred for 24 hours under anatmosphere of nitrogen. After being diluted with dichloromethane thereaction mixture was washed with sodium carbonate solution and water.After drying the dichloromethane was distilled off, and the residue waspurified on a column filled with silica gel and eluted with ahexane-ethyl acetate mixture (75/25). The yield was 20.8 g (66%) of4(2,3 epoxypropyl oxy)phenyl 4'methoxycyclohexyl carboxylate.

The epoxide of methoxycyclohexyl benzoate was used together with theepoxide of methoxyphenyl benzoate to prepare a liquid crystallinepolyether (LC 5) by using the general method for the synthesis of liquidcrystalline polyethers specified above, using methoxy phenyl-(2,3dihydroxypropyloxy) benzoate with an epoxy/OH ratio of 5:1. It appearedthat the cyclohexyl group containing epoxide was present for 16 mole %in the polyether.

Example 3

Procedure for making the retardation layers:

Used were two glass substrates of a thickness of 100 micrometers. Thesewere coated with Merck "LIQUICOAT®" PA (E. Merck, Fed. Rep. Germany),pre-cured at 60° C. for 15 minutes, cured at 300° C. for 1 hour, andthen rubbed in the appropriate direction on a felt cloth, in accordancewith the instructions provided by Merck. To ensure proper adhesion ofthe PI layer the glass substrates were cleaned in advance using thefollowing procedure:

ultra-sonic cleaning with a detergent (Q9, Purum GmbH)

KOH (1 M), 50°C./1 hr

HNO₃ /H₂ SO₄ /H₂ O (1:1:10), 60°C/1 hr

reflux in isopropyl alcohol vapour for 30 minutes or more.

Between each cleansing step a rinsing with demineralised water wasperformed. This is a variation on the method as described by W. H. deJeu in Physical properties of Liquid Crystals, 1st edition (Gordon andBreach Science Publishers), p. 23.

LC 3 was dissolved in cyclopentanone together with 5 wt. % of chiraldopant (Merck CB 15™). To the filtered solution 0.5 wt. % (calculated onLC material 3) of cross-linked polymer spheres (Dynospeheres DL 1060®,ex JSR) was added as spacers. The solution of LC material 3 with spacerswas spin-coated onto the two pretreated glass substrates. The layerthickness obtained was 4 micrometers. The two films of LC material 3were dried in a vacuum oven for 16 hours at 20° C. They were then placedone on top of the other under a 60° difference in orientation directionand moulded at a temperature of 160° C. Next, the sample was cooled to115° C., and after 5 minutes to room temperature. The quality of theresulting retardation film was determined with the aid of variousoptical techniques based on the theory described in E. P. Raynes, "TheOptical Properties of Supertwisted Liquid Crystal Layers", MolecularCrystals & Liquid Crystals Letters. 4(3-4) (1987), 69-75.

The dispersion of the high-molecular weight liquid-crystalline materialwas measured by fitting transmission spectra of the retardation layersbetween two polarisers to the formulae given in Raynes. In FIG. 2 thedispersion (defined as the retardation at a certain wavelength dividedby the retardation at 550 nm) was given for LC 3, a birefingentpolycarbonate film such as described in Jap. J. Appl Physics,V0l.30, No.4 (April 1991), 682-686, and a commercially available low-molecularweight liquid-crystalline active cell as used in the Sharp wordprocessorWD A 330™, and an active cell containing a commercially available liquidcrystal mixture ZLI 4544, ex Merck.

From FIG. 2 it can be seen that the dispersion of LC 3 according to theinvention is nearly the same as that of a commercially available activecell (a difference in dispersion of less than 0.1) over the wholewavelength area of 400-800 nm, whereas the dispersion of the 30birefringent polycarbonate film only matches that of the commerciallyavailable active cell at 550 nm , by definition, and shows largedeviations, especially in the shorter wavelength area of 400-550 nm.

The dispersion of LC 5 is nearly the same as that of the active cellcontaining ZLI 4544 over the whole wavelength area of 400-800 nm.

In FIG. 3 the dispersion is given for various LC materials according tothe invention. From FIG. 3 it can be seen that using mesogenic groupswith a by more conjugated system such as cyanobiphenyl gives a higherdispersion than when LC material having mesogenic groups with a lessconjugated system such as phenyl benzoate groups are used. A comparisonbetween LC material having nitrophenyl benzoate mesogenic groups and LCmaterial having methoxyphenyl benzoate mesogenic groups showed that thelatter, i.e., the least conjugated material, has the lowest dispersion.When replacing some of the phenyl groups for cyclohexyl groups in themesogenic groups, the dispersion is lowered even further. These examplesshow that the dispersion can be set by varying the mesogenic groups ofthe LC material.

We claim:
 1. A method for making a liquid-crystalline display, whichdisplay comprises an active liquid-crystalline cell and a retardationlayer comprising a high-molecular weight liquid-crystalline materialcomprising at least one mesogenic group, the method comprising adaptingthe dispersion of the retardation layer to that of the activeliquid-crystalline cell by varying the mesogenic group or groups of thehigh-molecular weight liquid-crystalline material, so that thedifference in dispersion between the active cell and the retardationlayer in the wavelength area of 400-800 nm is not more than 0.1.
 2. Themethod of claim 1, wherein the high-molecular weight liquid-crystallinematerial is placed between two orienting substrates, giving one of thesubstrates a different orientation direction from that of the othersubstrate.
 3. A liquid crystalline display made by the method ofclaim
 1. 4. The liquid-crystalline display of claim 3 wherein thehigh-molecular weight liquid crystallime material comprises at least onemesogenic group having a large conjugated system and at least onemesogenic group having a smaller conjugated system.
 5. The method ofclaim 1 wherein the difference in dispersion between the active cell andthe retardation layer in the wavelength area of 400-800 nm is not morethan 0.03.
 6. The method of claim 4 wherein the ratio of mesogenicgroups with large and small conjugated systems is varied.
 7. The methodof claim 1 wherein the dispersion is adapted by varying polarity of themesogenic group.
 8. The method of claim 1 wherein the mesogenic group ishalogenated.